The present disclosure relates generally to equipment utilized and operations performed in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides a control system for an annulus balanced subsurface safety valve.
A typical subsurface safety valve can be annulus-balanced, that is, one side of a piston of an actuator in the safety valve is exposed to pressure in an annulus surrounding the safety valve. In this type of safety valve, pressure is applied via a control line to an opposite side of the piston to displace the piston after overcoming a biasing force applied by a spring or other biasing device.
However, closing the valve requires that the fluid column in the control line be lifted by the biasing device. Thus, the biasing device must exert a large biasing force if the safety valve is set very deep in a well. In order to overcome this large biasing force, substantial pressure must be applied to the control line to open the valve. This requires a corresponding substantial pressure differential be applied across the piston. Also, flowing the fluid through the long control line takes substantial time when the valve is to be closed.
From the foregoing, it is apparent that improvements are needed in the art of controlling operation of a subsurface safety valve.